Conventional glassmaking methods require establishing in a glass melting furnace temperatures that are high enough to melt the glassmaking material (by which is meant one or more materials such as sand, soda ash, limestone, dolomite, feldspar, rouge, which are collectively known as “batch” and/or broken, scrap and recycled glass, known as “cullet”). The required high temperature is generally obtained by combustion of hydrocarbon fuel such as natural gas. The combustion produces gaseous combustion products, also known as flue gas. Even in glassmaking equipment that achieves a relatively high efficiency of heat transfer from the combustion to the glassmaking materials to be melted, the combustion products that exit the melting vessel typically have a temperature well in excess of 1093° C. (2000° F.), and thus represent a considerable waste of energy that is generated in the glassmaking operations unless that heat energy can be at least partially recovered from the combustion products. The prior art has addressed this problem by using flue gas to air heat exchangers known as regenerators. In a conventional air fired regenerative furnace, waste heat in the flue gas is partially recovered in the regenerators by preheating the incoming combustion air and the exit temperature of the flue gas after passing through the regenerators is reduced to about 427 to 538° C. (800 to 1000° F.).
Combustion of the hydrocarbon fuel with gaseous oxidant having an average of at least 35 volume percent oxygen (known as “oxy-fuel combustion”) provides to the glassmaking operation numerous advantages compared to combustion of the fuel with air. Among those advantages are higher flame temperature, which affords higher heat transfer and shorter melting times, and reduced overall volume of the gaseous combustion products that exit the glass melting furnace, which affords a reduction in the size of the gas-handling equipment that is needed. The gaseous combustion products formed in combustion with oxidants having such higher oxygen content can exhibit temperatures of 982° C. (1800° F.) or higher, even 1093° C. (2000° F.) or higher. Thus, the gaseous combustion products of oxy-fuel combustion contain even more heat energy, compared to the combustion products of conventional air-fired combustion, which should be used to advantage to improve the overall energy efficiency of the glassmaking operation.
While the glassmaking art is aware of using heat in the hot gaseous combustion products from the glass melting furnace to preheat incoming glassmaking material which is to be melted in the manufacture of the glass, the heretofore known technology has believed that the temperature of the hot combustion products should not exceed about 538 to 704° C. (1000 to 1300° F.) as it is fed commences heat exchange with the glassmaking material. This maximum temperature is imposed by considerations of the capability of the materials from which the heat exchanger is constructed to withstand higher temperatures, and considerations of the tendency of the glassmaking material to begin to soften and become adherent (or “sticky”) if it becomes too hot during the heat exchange step, leading to reduced throughput and even plugging of the heat exchanger passages. The temperature at which the glassmaking material becomes adherent or sticky depends on the batch composition and the material in contact with the glassmaking material and is believed to be in a range between 538 to 704° C. (1000 to 1300° F.) for a common batch to make soda lime glass for bottles and windows. In a conventional air fired regenerative furnace, the flue gas exit temperature after the regenerators is about 427 to 538° C. (800 to 1000° F.) and there is no need to cool down the flue gas prior to a batch/cullet preheater.
When the gaseous combustion products are those obtained by oxy-fuel combustion, the conventional belief has been that they need to be cooled to the range of from 538 to 704° C. (1000 to 1300° F.) before heat exchange with the incoming glassmaking materials can commence. Numerous examples exist showing the prior art's belief that the temperature of the flue gas must be reduced before the flue gas is used to heat incoming glassmaking materials. Such examples include C. P. Ross et al., “Glass Melting Technology: A Technical and Economic Assessment”, Glass Manufacturing Industry Council, August 2004, pp. 73-80; G. Lubitz et al., “Oxy-fuel Fired Furnace in Combination with Batch and Cullet Preheating”, presented at
NOVEM Energy Efficiency in Glass Industry Workshop (2000), pp. 69-84; U.S. Pat. Nos. 5,412,882; 5,526,580; and 5,807,418.
However, reducing the temperature of this stream of combustion products by adding to it a gaseous diluent such as air, and/or spraying a cooling liquid such as water into the stream, is disadvantageous as such approaches reduce the amount of recoverable heat remaining in the gaseous combustion products, increase the size of the gas handling equipment that is needed, and adds additional equipment and process expense.
Thus, there remains a need in this field for method and apparatus permitting practical and efficient heat exchange from the gaseous combustion products of oxy-fuel combustion to glassmaking material, which can be practiced even at the relatively higher temperatures encountered when using oxy-fuel combustion in glassmaking operations.